Near-field scanning optical microscopy (NSOM), as a surface imaging technology, has found its way into numerous applications including semiconductor and biological sample imaging, due to its subwavelength resolution that transcends the Rayleigh diffraction limit. A description may be found, for example, in Betzig et al., “Near-field optics: microscopy, spectroscopy, and surface modification beyond the diffraction limit,” Science, vol. 257, pp. 189-95, (1992), which is hereby incorporated by reference.
Unless the context requires otherwise, the term “near field,” as used herein and in any appended claims, substantively or adjectivally, refers to a regime in which evanescent (exponentially decaying) components of a scattering wave-function are significant. Other regions of the field, more distant from the defining aperture, are referred to herein as the “far field.”
Conventional NSOM takes the scattered optical intensity above a three-dimensional sample as a two-dimensional image of the sample. This interpretation of NSOM data suffers from ambiguity between variations in the topology and optical properties of the sample. Several works have addressed this ambiguity and the solution of a three-dimensional near-field inverse scattering problem (ISP) which makes possible three-dimensional tomographic imaging. The resulting methods are known as near-field scanning optical tomography (NSOT). Detailed discussion may be found in Sun et al., “Near field scanning optical tomography: a nondestructive method for three-dimensional nanoscale imaging,” IEEE J. Sel. Top. in Quant. Electronics, vol. 12, pp. 1072-82, (2006), and in Sun et al., “Strong tip effects in near-field scanning optical tomograpy,” Jour. Appl. Phys., vol. 102, 103103, (2007), both of which references are incorporated herein by reference.
It is to be understood that the term “aperture,” as used herein and in any appended claims, does not require an actual physical aperture, and that a sharp metal tip, for example, may serve as a pointlike secondary source that illuminates the sample and serves the function of an aperture for purposes of the present treatment, and is considered to be subsumed within the term “aperture” for current purposes. The “aperture” may be the facet of an optical fiber, possibly sharpened to reduce its characteristic dimension. Further discussion of the aperture may be found in Sun et al. (2007). Furthermore, unless the context otherwise requires, the term “near field,” as used herein and in any appended claims, substantively or adjectivally, shall have the meaning of a regime in which evanescent components of a scattering wavefunction are significant. Other regions of the field, more distant from the defining aperture, are referred to herein as the “far field.”
Current NSOT modalities require multi-directional measurements of the scattered field in illumination mode, or multi-angle illuminations in collection mode. Discussion of these modalities may be found in Sun (2006) and (2007). The requirements of multi-directional measurements in current NSOT modalities inevitably complicate NSOM experimental configurations. A simple way of obtaining more data with only minor changes of current experiments is to scan the probe over a three-dimensional volume above the sample, as described by Hillenbrand et al., “Complex optical constants on a subwavelength scale,” Phys. Rev. Lett., vol. 85, pp. 3029-32, (2000), incorporated herein by reference, which amounts to acquiring multiple NSOM images at different distances from the sample. Scanning a probe in three dimensions is discussed, as well, in Vogelsgang et al, “Beyond lock-in analysis for volumetric imaging in apertureless scanning near-field optical microscopy”, J. Microscopy, vol. 229, pt. 2, pp. 365-70, (2008), and in German Patent Application DE 10 2005 029 823.0, to Taubner et al., published Dec. 28, 2006, both of which references are incorporated herein by reference. In illumination or collection mode NSOM, such data sets are related by propagation or back-propagation of the scattered field, and are therefore considered redundant. See, Born & Wolf, Principles of Optics, 6th ed. (1980), reprinted (1997), which is incorporated herein by reference.